In the field of projection display, ultra short throw projection is increasingly wide-spreading for its conveniences in use. A projection apparatus includes four parts: a projection light source, an optical assembly, a lens and a screen, where the projection light source provides for the optical assembly with multi-primary-color (usually in three primary colors, i.e. red, green and blue, but may be other combinations as well) illumination light beams following a time sequence. When the light beams illuminate the core component of the optical assembly, i.e. a digital micro mirror device (DMD), the DMD will reflect, under the modulation of a display image signal, light beams of corresponding color components into the lens to form an image, where the lens of the projection apparatus is an ultra short-focus lens that includes an optic set and a reflector, to realize a low throw ratio through secondary imaging.
While projecting the image, it is desired that the light beams coming out of the projection light source into the optical assembly, the light beams being reflected by the DMD into the lens, and the light beams being reflected by the reflector all have uniform brightness, so as to alleviate nonuniform in the brightness of the projection image.
However, when an image is projected, the brightness may appear to be nonuniform. This phenomenon is due to the fact that the capacity of the optical lenses to reform and homogenize the light beams is limited by their craftsmanship, as well as accumulated tolerances in assembling and machining of each lens in the system architecture. When such nonuniformity is magnified and displayed onto the projection screen, a user will notice unwanted contrasts of brightness against darkness within image regions on the screen, leading to deteriorated projection image quality.